1. Field of the Invention
The invention generally relates to the detection of defects in semiconductor substrates. More particularly, the invention relates to the detection and localization of cracks in semiconductor substrates, such as in wafers.
2. Description of Related Art
For detecting cracks in semiconductor substrates, mechanical methods are known, in particular tactile and acoustic methods. For example, it is known from WO 2008/112597 A1 to compare the resonant frequencies of a silicon wafer with the resonant frequencies of a reference wafer. Deviations of the resonance frequencies are then considered to be indicative of cracks. A disadvantage herein is the mechanical stress that may possibly result in breaking, even of an intact wafer.
Moreover, optical methods are known. For example, the apparatus described in DE 01 46 879 A1 is based on a backlight arrangement. The wafer is irradiated with light at one side. On the opposite side a camera is arranged which detects the light passing through a crack. However, a requirement herein is that the crack extends through the wafer or is at least so deep that light can penetrate through the crack.
Other optical methods that use a similar arrangement are likewise based on a bright-field observation, wherein the wafer is irradiated with light at its rear face, which light, however, is transmitted through the wafer due to its wavelength for which the wafer is transparent. The wafer is then optically analyzed on the opposite side using a camera. At a crack, transmittance slightly varies so that the crack can be detected by the camera. Such an arrangement is known from JP 08220008 A. However, a drawback thereof is that the signal of transmittance variation is superimposed by a much stronger background signal in form of the light normally transmitted through the wafer. This makes detection difficult, especially for small cracks.